Contamination during electron beam imaging and analysis in SEMs and similar devices comes primarily from a sample under test (SUT). When electrons bombard the sample, they cause materials from the sample to be ejected. These materials can have very long mean free paths in vacuum and therefore can deposit on surfaces located at great distances from their origin. The most common contamination is polymer that comes from samples containing photoresist or other resins common to integrated circuit processing. The lenses in a scanning electron microscope (SEM) are typically in line-of-site to the SUT and exposed to any ejected polymer. The lenses are typically held at high voltages and are immersed in a low energy electron cloud. When polymer, being a dielectric, deposits on these lenses and is exposed to these conditions, it charges and causes distortions. Eventually the electron beam column will need to be cleaned or replaced. The situation is more severe in low-vacuum or environmental SEMs where the vacuum pressure near the sample can be up to ˜10 Torr.
Accordingly, a new apparatus and method are needed for reducing contamination in a SEM and other charged particle devices.